Rotational pump and methods for controlling rotational pumps

ABSTRACT

A rotational pump capable of running at a rotational speed (n) having a system for direct or indirect measurement of pressure difference or flow rate across the pump, wherein a control system is designed to calculate an index of pulsatility (PI) of the pressure difference or flow rate, estimating the gradient of PI with respect to the rotational speed (dPI/dn) and regulating the dPI/dn to a pre-defined set-point or regulating the pump in a way that the dPI/dn is minimal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35 USC§120 to, U.S. Non-Provisional patent application Ser. No. 12/671,945,entitled “ROTATIONAL PUMP AND METHODS FOR CONTROLLING ROTATIONAL PUMPS,”which is a 371 nationalization of PCT/EP2008/006510 filed Jan. 1, 2008,the entire contents of which are incorporated herein by reference, andwhich, in turn, claims priority under 35 USC §119(e) to U.S. ProvisionalApplication 60/953,772 filed Aug. 3, 2007 and priority under 35 USC §119to European Patent Application 07075665.5 filed Aug. 3, 2007.

BACKGROUND

The invention relates to rotational pumps and methods for controllingrotational pumps.

Although research in the field of physiological control of rotary pumpsdates back to the early 1990s rotary blood pumps (RBP) used as leftventricular assist devices (LVADs) were initially operated at a constantrotational speed which was adjusted individually according to thepatient's need. Early clinical experience clearly showed thatventricular collapse and excessive suction are serious hazards relatedwith the operation of these pumps.

These rotational blood pumps are implanted into a human body. The inletof the blood pump is normally connectable to the left ventricle of thehuman, the outlet of a pump is connectable to the aorta, downstream ofthe aortic valve (AoV).

RBPs used as LVADs are often required to deliver the maximum possibleflow rate. This may be the case in the early post-op period or whenseriously impaired end-organ function requires optimum perfusion.Several approaches are known that attempt to meet this requirement byoperating the pump near the collapse point of the LV, where the flowrate is as high as possible. On the other hand, it is known thatexcessive unloading of the LV may impair the pumping performance of theright ventricle because of the septum shift. Furthermore, it ishypothesized that the alteration of the natural flow path of the LV incombination with the greatly reduced LV wall movement due to fullunloading causes recirculation and stasis inside the LV cavity. To date,there is only anecdotal evidence of thrombus formation in the LV, butatrial fibrillation can be considered to be a comparable situation inwhich thrombo-embolic complications are a well-known problem.Additionally, full unloading is contra-indicated for patients whosehearts may recover under assist and who are potential candidates forweaning. These facts strongly indicate that it may be better not alwaysto operate the RBP at the point of maximum flow rate but also at a pointwhere unloading is only partial, LV volume and LV wall movement are notminimal and at the optimum achievable washout of the LV cavity and wherethe aortic valve opens at least occasionally.

It is the object of the invention to provide a rotational blood pump anda control method which finds and adjusts the optimum operating pointunder all conceivable physiological situations without requiring theattention of a physician. An operating point may be optimal with regardto the therapeutic objectives mentioned above and which can beclassified into two cases: Full Assist (FA)—maximum support with closedAoV but sufficient safety margin to avoid suction, and Partial Assist(PA)—moderate support at the transition region between the opening ofthe AoV and a permanently-closed AoV with near-physiological LV volume,better LV washout and moderate LV loading.

SUMMARY

A rotational pump, especially a rotational blood pump capable of runningat a rotational speed (n) having a system for direct or indirectmeasurement of pressure difference or flow rate across the pump, whereina control system is designed to calculate an index of pulsatility (PI)of the pressure difference or flow rate, estimating the gradient of PIwith respect to the rotational speed (GPI=dPI/dn) and regulating the GPIto a pre-defined set-point or regulating the pump in a way that the GPIis minimal.

A method to control a rotational blood pump, characterized by direct orindirect measurement of the pressure difference or flow rate across thepump, calculating an index of pulsatility (PI) of the pressuredifference or flow rate, estimating the gradient of PI with respect torotational speed (n) GPI and regulating the GPI to a pre-definedset-point or regulating the pump in a way that the GPI is minimal.

The pump might be used in different technical fields. It is advisable toimplant the rotational blood pump into a human or animal body whereinthe inlet of the rotational pump is to be connected with the leftventricle of the heart and the outlet of the pump is to be connectedwith the aorta, downstream of the aortic valve. It is also conceivableto implant the pump as a right ventricular assist device (RVAD), wherethe inlet of the pump is connected to the right ventricle and the outletis connected to the pulmonary artery, downstream of the pulmonary valve.For simplicity, only the LVAD case shall be described below, withoutlimiting the invention to LVAD.

A control strategy for rotary blood pumps meeting differentuser-selectable control objectives is proposed: maximum support with thehighest feasible flow rate versus medium support with maximumventricular washout and controlled opening of the aortic valve. Apulsatility index (PI) is calculated from the pressure difference, whichmight be deduced from the axial thrust measured by a magnetic bearing ofthe pump or by other means. Alternatively the flow rate through the pumpmay serve as the basis for calculating the PI. The gradient of PI withrespect to pump speed (GPI) might be estimated via on-line systemidentification. The outer loop of a cascaded controller regulates GPI toa reference value satisfying the selected control objective. The innerloop controls the PI to a reference value set by the outer loop. Adversepumping states such as suction and regurgitation can be detected on thebasis of the GPI estimates and corrected by the controller. Alumped-parameter computer model of the assisted circulation may be usedto simulate variations of ventricular contractility, pulmonary venouspressure and aortic pressure. The performance of the outer control loopmay be demonstrated by transitions between the two control modes. Fastreaction of the inner loop may be tested by stepwise reduction of venousreturn. For maximum support, a low PI may be maintained without inducingventricular collapse. For maximum washout, the pump may work at a highPI in the transition region between the opening and the permanentlyclosed aortic valve. The cascaded control of GPI and PI is able to meetdifferent control objectives.

The gradient GPI is extracted from the system dynamics which isidentified by an on-line parameter estimation method.

The set-point of GPI may be selected in such a way that the pumpoperates in the transitional phase in between an opening an a closedaortic valve, this phase being at the transition point between partialand total assist.

The GPI might be regulated to the set-point by a cascaded controllerwith an inner and an outer loop. The outer loop may comprise a feedbackcontrol loop that keeps the GPI to its set-point and whose output is areference value for the PI.

An inner feedback control loop may keep the actual PI close to thereference value for PI by calculating a reference value for therotational speed.

The minimum of GPI maybe maintained by a cascaded controller with aninner and an outer loop.

The outer loop may comprise a feedback control loop that keeps the GPIto its minimum value and whose output is a reference value for the PI.

The inner feedback control loop of the controller may keep the actual PIclose to the reference value for PI by calculation a reference value forthe rotational speed.

The rotational speed may be temporarily reduced from the operating pointby a fixed value to allow the aortic valve (or pulmonary valverespectively) to open in systole.

The parameters of the inner feedback control loop may be adapted to theestimated system dynamics.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in the following:

FIG. 1 is a lumped-parameter simulation model, p_(ven), pulmonary venouspressure; L_(ven), R_(ven), inertia and resistance of pulmonary vein;E_(aa), E_(ap), active and passive left atrial elastance; L_(mit),R_(mit), inertia and resistance of mitral valve; D_(mit), mitral valve;E_(va), E_(vp), active and passive left ventricular elastance; R_(v),left ventricular viscous element; E_(vs), left ventricular serieselastance; D_(ao), aortic valve; R_(ao), resistance of aortic valve;Z_(c), L_(a), E_(sys), R_(sys), 4-element windkessel model of aorta withcharacteristic impedance, inertia, elastance and systemic resistance ofthe aorta; p, pressure difference generated by the assist pump; L_(k),R_(k), E_(k), inertia, resistance and elastance of the cannulas.

FIG. 2 shows a simulated pressure waveforms for aortic pressure AoP=85mmHg, contractility E_(max)=1 mmHg/ml, pulmonary venous pressurep_(ven)=8 mmHg and pump speed ω=7500 rpm. LVP, left ventricularpressure; LAP, left atrial pressure; AoP, aortic pressure; Δp, pressuredifference across assist pump; AoF, aortic flow; PF, pump flow.

FIG. 3 shows the dependence of pulsatility index PI and its gradient GPIon pump speed ω for different pulmonary venous pressures p_(ven).

FIG. 4 shows the dependence of pulsatility index PI and its gradient GPIon pump speed ω for different contractility levels E_(max).

FIG. 5 shows the dependence of pulsatility index PI and its gradient GPIon Pump speed ω for different levels of mean aortic pressure AoP.

FIG. 6 is a schematic control loop, ω, pump speed; Δp, pressuredifference; PI, pulsatility index; PI*, reference PI; GPI, gradient ofPI with respect to ω; DRBS, discrete random binary signal; ω_(E),auxiliary sinusoidal signal.

FIG. 7 is a block diagram of GPI control (outer control loop) Left:extremum seeking control for mode FA; Right: Reference tracking controlwith gradient information. HP, high-pass; ∫, integrator; ω_(E),auxiliary sinusoidal signal; Plant, pump with left heart and adjacentvasculature.

FIG. 8 shows a schematic of inner control loop for regulation of PI.

FIG. 9 shows the transition from mode FA to mode PA.

FIG. 10 shows the transition from mode PA to mode FA.

FIG. 11 shows pressure waveforms for both operation modes. Left: ModeFA; Right: Mode PA.

FIG. 12 is a transient response to a step decrease of p_(ven).

FIG. 13 is a rotational blood pump.

FIG. 14A rotational blood pump connected to a human heart.

DESCRIPTION OF PREFERRED EMBODIMENTS

A lumped-parameter computer simulation model was developed to design andtest the control algorithm (FIG. 1). The model consists of the pulmonaryvenous vasculature, left atrium (LA), LV, aorta and assist pump. Thepulmonary veins, represented by inertia L_(ven) and viscous elementR_(ven), are supplied by the pulmonary venous pressure p_(ven). The LVwas modeled by an E(t)-R model incorporating an active time-varyingelastance E_(va)(t), a pressure-dependent viscous element R_(v) andseries elastance E_(vs). The exponential relationship between passivefilling pressure and filling volume Q_(v) is accounted for by thepassive elastance E_(vp)(Q_(v)). The LA is modeled using a simpler E(t)model with active elastance E_(aa)(t) and constant passive elastanceE_(ap) according to a linear passive pressure-volume relationship. Theopen mitral valve is simulated by inertia L_(mit) and viscous termR_(mit), whereas the open aortic valve has only a viscous term R_(ao).Both valves, when dosed, are simulated by infinite resistances. Theaorta is represented by a 4-element windkessel model consisting of thecharacteristic impedance Z_(c) and inertia La of the proximal aorta,elastance E_(sys) and peripheral resistance R_(sys). A model of theINCOR axial-flow blood pump (Berlin Heart GmbH, Berlin, Germany) wasused as an assist pump model. The pump characteristic Δp=f({dot over(Q)}_(p), ω) is approximated by a multiple regression model withpressure difference Δp, pump flow rate {dot over (Q)}_(p) and pump speedω. The cannulas of the INCOR system are represented by an inertial termL_(k), viscous term R_(k) and elastic term E_(k). The whole network canbe described by a set of 9 non-linear first order differential equationswith state vectorx=[{dot over (Q)} _(ven) p _(ap) {dot over (Q)} _(mit) p _(vp) p _(vs){dot over (Q)} _(p) p _(ao) Q _(L) p _(sys)]^(T),  (1)where {dot over (Q)}_(ven) is the pulmonary venous flow, p_(ap) thepassive LA pressure, {dot over (Q)}_(mit), the trans-mitral flow, p_(vp)the passive LV pressure, p_(vs) the auxobaric LV pressure, {dot over(Q)}_(p) the pump flow, p_(ao) the proximal aortic pressure, {dot over(Q)}_(L) the proximal aortic flow and p_(sys) is the systemic arterialpressure. The control vector isu=[p _(ven) E _(aa) E _(va)ω]^(T).  (2)No output vector shall be defined for simulation purposes, as all statescan be monitored.

The elastance functions E_(aa)(t) and E_(va)(t) resemble the atrial andventricular activation functions. They can be normalized with respect totime and magnitude with max(E_(N)(t_(N)))=1 for t_(N)=1. The normalizedelastance function was approximated by a hybrid cosine function:

$\begin{matrix}{{E_{N}\left( t_{N} \right)} = {0.5\begin{Bmatrix}{1 - {\cos\;\left( {\pi\; t_{N}} \right)}} & {{{for}\mspace{14mu} 0} \leq t_{N} \leq 1} \\{1 + {\cos\left( \frac{t_{N} - 1}{t_{end} - 1} \right)}} & {{{for}\mspace{14mu} 1} \leq t_{N} \leq {t_{end}.}}\end{Bmatrix}}} & (3)\end{matrix}$with t_(end)=15, a relaxation time between 50 and 80 ms can be achievedfor auxobaric contraction.

Occlusion of the input cannula due to negative left ventricular pressure(LVP) was implemented by setting R_(v)=∞ for LVP<1 mmHg. A smallhysteresis reproduces the characteristic suction limit cycles observedin patients.

The model was implemented with Matlab/Simulink (The MathWorks, Natick,Mass., USA). All physiological parameters have been set according toliterature data and the pressure and flow waveforms have been validatedwith literature data as well. The pressure difference waveform wascompared to patient data from the INCOR patient database. FIG. 2 showsthe pressure and flow waveforms for simulation of an assistedpathological left ventricle.

Note that the input ω is the only one of the 4 elements of controlvector u directly accessible in clinical use, whereas p_(ven), E_(aa)and E_(va) are unknown. The output vector contains only the measurablevariables pump flow and pressure differencey=[{dot over (Q)} _(p) Δp] ^(T).  (4)

If the system were to be linearized at a certain operating point, itwould not be completely observable because most elements of state matrixA and most of the control signal values are unknown. The proposedcontrol strategy is based on regulating the LV pressure, orcorrespondingly, the filling volume Q_(v) which, on account of thenonlinearities, is reflected by the PI of the pressure difference signalfor a given contractility E_(max) and afterload AoP. The PI is filteredout of the pressure difference signal (provided by the magnetic bearingof the pump) by low-pass filtering (LP) of the magnitude (abs) of thehigh-pass filtered (HP) Δp signal:PI=LP{abs[HP(Δp)]}.  (5)

The dependence of PI on ω for different filling pressures is shown inthe top part of FIG. 3. For ω<ω_(PA), PI remains almost constant at ahigh level because the AoV opens in every systole. For ω_(PA)≦ω≦ω_(S),the AoV remains permanently closed and PI decreases with increasing ω.When the minimum is reached, suction starts due to a low LVend-diastolic volume and low LV end-diastolic pressure. For ω>ω_(S) PIincreases again, caused by the positive suction spikes of the pressuredifference. It can be seen that higher filling pressures shift the PIcurves to higher ω values. Independently of p_(ven), optimal operatingpoints can be assigned: for PA mode this is ω_(PA), whereas the maximumnegative slope of the PI, marked as ω_(FA), was selected for FA mode. Atω_(FA), a high pump flow is achieved with a sufficient safety marginwith respect to suction. To determine both these operating points, thegradient of PI with respect to ω(GPI=δPI/δω)) was calculated off-line(FIG. 3, bottom). If the operating points ω_(PA) are transferred to theGPI, it can be seen that these points are all located at a smallnegative value of GPI, regardless of p_(ven). The points ω_(FA) arelocated at the minimum of GPI. This relation is also true for differentlevels of contractility (FIG. 4) and afterload (FIG. 5). The controltask consists of determining and tracking these operating pointson-line.

A cascaded control loop was designed (FIG. 6). The outer loop regulatesthe GPI according to the selected operating mode. A parameter estimationalgorithm calculates the current GPI using present and past values ofthe plant input ω′ and plant output PI, where ω′ is the reference pumpspeed ω superimposed with an auxiliary discrete random binary signal(DRBS) of small amplitude. The process is assumed to be linear in thevicinity of the current operating point and to be time-varying. Thelinear time invariant discrete-time difference equation of an ARXprocess model of order tri and delay d is given byy(k)+a ₁ y(k−1)+ . . . +a _(m) y(k−m)=b ₁ u(k−d)+ . . . +b _(m)u(k−d−m)+e(k)  (6)

with inputs u=ω, outputs y=PI and equation error e which must be assumedto be white noise. A recursive least square (RLS) method estimates thesystem parameters a₁ . . . a_(m) and b₁ . . . b_(m) on-line. The GPI canbe calculated as the plant gain:

$\begin{matrix}{{GPI} = {\frac{\sum\limits_{k = 1}^{m}\; b_{k}}{1 + {\sum\limits_{k = 1}^{m}\; a_{k}}}.}} & (7)\end{matrix}$

The system parameters may vary slowly or rapidly with time. Slowlytime-varying parameters can be tracked with a constant forgetting factorapproach with sufficiently low parameter variance. This may be the casefor gradual changes of venous return, afterload or contractility.Rapidly varying or jumping systems, however, require special algorithmsto allow fast tracking without sacrificing smoothness of the estimates.A sudden change of venous return may occur during a change of bodyposture and when straining or coughing. A time-varying forgetting factorapproach which is controlled by the a-posteriori variance of theestimation error was used.

Extremum-seeking control (ESC) is employed for controlling the GPI (FIG.7). ESC minimizes the objective function GPI=f(ω). As GPI(ω) is a convexfunction for GPI<0 (see FIG. 3, bottom), the minimum can be found. ESCrelies on auxiliary excitation of the plant input signal ω. The cascadedcontroller as shown in FIG. 6 however, allows no direct manipulation ofω by the gradient controller. Instead, the reference value PI* must beused to impose the required excitation signal. As PI=f(ω) is amonotonically falling function for GPI<0, GPI=f(PI) is also a convexfunction. The excitation signal is a sine wave with low frequency andamplitude. This signal is also used to demodulate the high-pass-filteredplant output to extract the gradient information which is then fed intoan integrator. The output of the integrator approaches the PI* value forwhich GPI is at a minimum (i.e. δGPI/δPI=0).

In PA mode, the current estimate of the GPI is kept at a constantnegative reference value (e.g. −3 mmHg·min) by an integral controller.The ESC is merely used to extract gradient information to detect thefalling slope of the function GPI=f(ω) (which corresponds to the risingslope of GPI=f(PI)). If an incorrect slope is detected, the mode istemporarily switched to FA until the extremum is found. Following afurther increase of PI* (reduction of ω) the mode is switched back to PAmode.

Adverse pumping states such as suction and regurgitation can be detectedon the basis of non-negative GPI estimates and are corrected by thecontroller. The output PI* of the gradient controller is the referencesignal for the inner control loop (FIG. 8). A predictive controller wasdesigned using the Internal Model Control (IMC) scheme. The plant in (6)may be writtenA(q ⁻¹)y(t)=q ^(−d) B(q ⁻¹)u(t)  (8)

with the polynomials A(q⁻¹)=1+a₁q⁻¹+ . . . +a_(m)q^(−m) andB(q⁻¹)=b(q⁻¹)=b_(1q) ⁻¹+ . . . +b_(m)q^(−m). The transfer function canbe derived from Eq. 8

$\begin{matrix}{{G\left( q^{- 1} \right)} = {\frac{q^{- d}{B\left( q^{- 1} \right)}}{A\left( q^{- 1} \right)}.}} & (9)\end{matrix}$

The closed-loop poles P contain the poles of the plant A and theauxiliary poles P₀:P(q ⁻¹)=A(q ⁻¹)P ₀(q ⁻¹).  (10)

Polynomial T is used to design the tracking dynamics. Polynomials T andP₀ were properly tuned to yield robust stability and performance forvarying plant gains (i.e. varying GPI).

The behavior of the entire control loop was tested in simulations forvarious combinations of physiological parameters. Unless otherwisestated, a typical parameter set was used as a standard for all followingsimulations: E_(max)=1 mmHg/ml, AoP=85 mmHg, p_(ven)=4 mmHg and heartrate=90 bpm.

Simulations were carried out for both modes of operation. FIG. 9 showsthe transition from mode FA to PA to demonstrate the performance of theouter control loop for GPI control. The GPI changes from −11 mmHg·min tothe required −3 mmHg·min within 500 s. Within the same time, PI risesfrom 12 mmHg to 22 mmHg, ω decreases from 7700 rpm to 6400 rpm and PFdecreases from 5.0 l/min to 3.5 l/min. Note that the reduction of PF ina patient would force p_(ven) to rise, causing a recovery of PF at theexpense of a higher left-ventricular volume (LVV) and higher LAP. In thesimulation, however, p_(ven) has been kept constant. FIG. 10 shows thetransition back to FA. GPI, PI, ω and PF revert to their originalvalues. This transition takes roughly 1000 s. In both operating modes,the gradient control loop was stable. The oscillations of PI, ω and PFare caused by the sinusoidal excitation for ESC. In FA mode, the levelof PF was high enough to keep the LVV in a range where the LVP stayswell below the aortic pressure (FIG. 11, left). In PA mode, the LVPbriefly reaches the level of AoP which allows the AoV to open (FIG. 11,right). The peak LVP oscillates periodically due to the sinusoidalexcitation.

The reaction of the inner control loop to sudden changes in venousreturn is simulated by a stepwise decrease of p_(ven) from 6 mmHg to 4mmHg (FIG. 12). FA mode is the more demanding test case because thesafety margin to the suction point is smaller than in PA mode. The pulseamplitude of the Δp signal drops to zero within 2 heart beats. Onesuction spike occurs before ω is rapidly reduced and further suctionspikes can be avoided. The PI recovers within 10 s and increases toabove the initial value. After another 15 s (not shown in FIG. 12), thePI returns to the initial value. The pump speed decreases from 9660 rpmto 7375 rpm. In PA mode (not shown), no suction spikes occur during astep decrease of p_(ven).

Preload-based control of RBP is the most common control method used forclinically available pumps as well as for investigational devices.Preload is reflected in the pulsatility of the PF, Δp or motor currentsignal, provided that the LV demonstrates some residual contractility.Methods based on maintaining a predetermined PI reference leveldemonstrated a lack of adaptation to changing physiological variablessuch as contractility or afterload since the level of PI is affected bythese variables. Consequently, the adjusted reference value for the PIis optimal only for one particular parameter set. If contractilityincreases, for instance, the PI would have to be increased too. Thereare several approaches to address this problem. It may be possible topropose a manipulation of the pump speed to verify whether somecharacteristics as PI, pump flow and power consumption behave asexpected. As a result of the speed changes, the reference value for PIis increased when there is an imminent risk of suction, or otherwisedecreased. Our proposed control method is also based on the applicationof speed variations for evaluating the reaction of the system. Wecontinuously obtain an estimate of the GPI which can be used to fulfildifferent control purposes by simply controlling the GPI in anappropriate way.

Several proposed control methods aim to adaptively operate the pump at apoint where the flow rate is maximal. It may be possible to increase thepump speed until the point of suction is detected and subsequentlydecrease the pump speed. This approach has been further investigated bythe same group. A method based on ESC has been proposed to maximizeeither the mean PF or the PF during diastole. Consequently, the pump isoperated near the collapse point of the LV. To increase the safetymargin to the suction point, slope-seeking control, as a special case ofESC, has been proposed in order to operate the pump at a slightly higherdegree of LVV. Similarly, it might be tried to use own nor pressurepulsatility on its own as a control variable, but rather the quotient ofthese two parameters. This index will decrease at induction of suctionand can thus discriminate between pulsatility caused by LV contractionand suction. All these methods have in common that the pump is operatednear the onset of suction. Occasional occurrence of suction is eventolerated. It is our opinion, however, that suction has to be avoidedunder all circumstances. Unlike the strategies outlined above, wepropose to operate the pump in FA mode at a speed where ventricularcollapse is unlikely due to the larger safety margin towards suction. Itis not necessary to test for the onset of suction and suction cantherefore be avoided.

In contrast to the high-flow operating point, with our PA mode, weadditionally propose a method which can operate the pump at a pointwhere the degree of unloading is not maximal, but where the LV fillingis more physiological and ventricular washout is optimized due to betterLV wall movement. We defined this point to be in the transition regionbetween the point where the aortic valve opens and that point where itis permanently closed. This operating point is often selected manuallyby the physician, either with echocardiographic guidance or byinterpreting the pressure difference waveform. This region can bedetected quite precisely using the gradient information of PI withrespect to pump speed (GPI), provided that the residual contractility ofthe LV is high enough at all to enable ejection through the AoV at lowpump speeds. However, the GPI is not readily available when the pump isoperated at one specific speed. It can be estimated with parameterestimation methods based on observation of input-output data over acertain time interval. For proper excitation of the system, an auxiliarysignal (DRBS) has to be added to the input. The resulting speed changesare not expected to be perceived by the patient. We applied ESC tocontrol the GPI. ESC also needs an auxiliary signal which has a muchlower frequency but a higher amplitude than the DRBS. The resultinglow-frequency oscillation might be a drawback of this method, but hasthe positive side effect that in PA mode the AoV will open duringlow-speed phases and stay closed during high-speed phases. The ratherlong response time of the ESC is based on the recursive estimation time.The gradient control loop determines the proper reference signal forcontrol of PI. This reference point has to be modified according to thechanging physiological parameters. Changes of E_(max) require thelargest corrections of the reference point (see FIG. 4), followed bychanges of AoP (see FIG. 5). As the contractility is not expected tochange suddenly, the adaptation velocity of the controller is believedto be adequate. Changes in AoP caused by changes of the systemicvascular resistance (SVR) are usually ramp-like changes. If, forexample, the AoP decreases too fast for the PI* follow suit when in PAmode, pump speed will temporarily decrease until the controller reactsby decreasing PI*. The fastest changes are expected for p_(ven).However, PI* can be kept almost constant for altering p_(ven) (see FIG.3).

Sudden changes in p_(ven) are handled instead by the inner control loop.The applied IMC scheme, as a special case of pole placement strategy, isa simple control structure which has the advantage of an easy design ofthe closed-loop poles to achieve fast regulation dynamics withoutovershoot. If the closed-loop dynamics are not faster than the open loopdynamics, the IMC scheme inherently offers a convenient way to ensurepredictable behavior in the presence of input constraints (speed limitsof the pump). The response to output disturbances is fast enough toavoid collapse of the LV. In FA mode, only one suction spike occurs whenthe venous return is suddenly reduced. Such a fast transient has notbeen observed in any patient according to the INCOR patient data base.Generally, the pulse amplitude does not drop to zero any faster thanwithin 5 consecutive heart beats. Hence, the simulated reduction within2 heart beats can be regarded as being the worst case. Almost noovershoot has been observed, neither for reference nor for output stepresponses at different values for GPI. The fast dynamics of the IMC hadto be traded against a slow reaction during arrhythmias. Although goodrobustness against arrhythmias is anticipated due to the use oftime-averaging algorithms rather than pattern recognition methods, testswith various forms of arrhythmia still have to be carried out.

The physician is given the option of selecting between full assist andpartial assist. The reduction to just two distinct options may seemdraconic, but the objective is to relieve the physician from having tomake decisions on to many poorly-known variables.

For a deeper understanding of the invention's rotational pump. FIG. 13shows a schematic view of a rotational pump.

FIG. 14 shows this pump connected to a human heart.

The invention claimed is:
 1. A method to control a rotational blood pump operating at a rotational speed comprising: selecting a user-selectable control objective from at least two control objectives; measuring at least one of a pressure, a pressure difference or a flow rate; regulating the pump to an operating point satisfying the selected control objective by modifying the rotational speed of the pump based on the at least one of the pressure, the pressure difference or the flow rate, calculating an index of pulsatility from the pressure difference or the flow rate; and controlling the index of pulsatility to a set point based on a gradient of the index of pulsatility, wherein the modifying the rotational speed of the pump comprises the regulating the index of pulsatility, and wherein the rotational speed of the pump is temporarily reduced from the operating point to allow an aortic valve or a pulmonary valve to open in systole.
 2. A method to control a rotational blood pump operating at a rotational speed comprising: selecting a user-selectable control objective from at least two control objectives; measuring one of a pressure, a pressure difference and a flow rate; and regulating the pump to an operating point satisfying the selected control objective by modifying the rotational speed of the pump based on the measured one of the pressure, the pressure difference and the flow rate, wherein an index of pulsatility is calculated and the index of pulsatility is calculated from the pressure difference or flow rate, and wherein the index of pulsatility is set to a reference value depending on a gradient of the index of pulsatility.
 3. The method according to claim 2, wherein the operating point corresponds to the reference value of the gradient of the index of pulsatility, said gradient being selected in such a way that the pump operates in a transitional phase in between an opening and a closing of an aortic valve or a pulmonary valve.
 4. The method of claim 2, wherein the gradient of the index of pulsatility is determined by means of random speed variations and a recursively amended model.
 5. The method of claim 2, wherein a model for calculating the gradient of the index of pulsatility is used, which is adapted with a changing rate that depends on a patient's activity.
 6. The method of claim 2, wherein the gradient of the index of pulsatility is controlled to its minimum by an extremum controller.
 7. The method of claim 2, wherein modifying the rotational speed of the pump is based on the index of pulsatility and the measured one of the pressure, the pressure difference and the flow rate.
 8. A method to control a rotational blood pump operating at a rotational speed comprising: selecting a user-selectable control objective from at least two control objectives; measuring one of a pressure, a pressure difference and a flow rate; regulating the pump to an operating point satisfying the selected control objective by modifying the rotational speed of the pump based on the measured one of the pressure, the pressure difference and the flow rate, calculating an index of pulsatility from the pressure difference or the flow rate; and setting a reference value of the index of pulsatility depending on a gradient of the index of pulsatility, wherein the operating point is selected in such a way that the pump operates in a transitional phase in between an opening and a closing of an aortic valve or a pulmonary valve.
 9. The method according to claim 8, wherein the control objectives include a partial assist mode and a full assist mode.
 10. A method to control a rotational blood pump operating at a rotational speed comprising: selecting a user-selectable control objective from at least two control objectives; measuring at least one of a pressure, a pressure difference or a flow rate; regulating the pump to an operating point satisfying the selected control objective by modifying the rotational speed of the pump based on the at least one of the pressure, the pressure difference or the flow rate, calculating an index of pulsatility from the pressure difference or the flow rate, and wherein regulating the pump to the operating point comprises regulating the index of pulsatility to a reference value based on a gradient of the index of pulsatility, wherein a preload-based control is used to verify whether a pre-defined set-point is to be increased or decreased.
 11. A control system for controlling a rotational pump capable of running at a rotational speed, the control system comprising a measuring system coupled to the pump for direct or indirect measurement of at least one of a pressure, a pressure difference or a flow rate across the pump, the control system configured to regulate the pump to an operating point, wherein the operating point is selected such that the pump operates in a transition region between an opening and a closed aortic valve, the region being at a transition point between a full assist mode and a partial assist mode, wherein the control system is configured to temporarily reduce the rotational speed of the pump in the transitional region by or to a fixed value to allow an aortic valve or a pulmonary valve to open in systole, wherein the control system is configured to calculate an index of pulsatility from the pressure difference or the flow rate, and wherein the control system is configured to temporarily reduce the rotational speed of the pump in the transitional region based on a regulation of the index of pulsatility to a reference value, wherein the regulation of the index of pulsatility is dependent on a gradient of the index of pulsatility.
 12. The control system of claim 11, wherein the control system is configured to temporarily reduce the rotational speed of the pump in the transitional region by the fixed value to allow the aortic valve or the pulmonary valve to open in systole. 